Publications
This research could not be possible without fruitful academic and industry collaborations. A special thanks goes to Benedikt Schwarz's group at TU Wien for device fabrication.
Our recent works are supported by the NDSEG Fellowship Program, DRS Daylight Solutions, and the National Science Foundation under Grant No. ECCS-2221715.
Under Review, PDF
Theodore P. Letsou*, Johannes Fuchsberger*, Nikola Opačak, Dmitry Kazakov, Benedikt Schwarz, and Federico Capasso
Abstract: High-power, broadband frequency combs generated by semiconductor lasers have profound implications for on-chip spectroscopy. Here, we present a dry-etched racetrack quantum cascade laser that uses resonant radio-frequency injection to produce a unidirectional quantum walk frequency comb at mid-infrared wavelengths. Efficient light outcoupling from the racetrack resonator provides more than 100 mW of continuous-wave output power at room temperature, with beam quality on par with that of Fabry--Perot lasers. Experimental waveform reconstruction confirms that the combs are frequency-modulated, rather than amplitude-modulated (as in active mode-locking). We show excellent agreement between the experimental waveforms and numerical simulations, which are based on a reduced Maxwell--Bloch model of the laser and include fast gain dynamics, finite group velocity dispersion, and large Kerr nonlinearity. Furthermore, our optimized device architecture -- featuring thick Si3N4 passivation and reduced parasitic capacitance -- enables modulation bandwidths exceeding 10 GHz. Combined with high output powers and the potential for monolithic integration of multiple ring lasers and waveguide couplers, these advances pave the way to fully integrated dual-comb spectrometers.
Nature 641, 83-89, PDF
Dmitry Kazakov*, Theodore P. Letsou*, Marco Piccardo, Lorenzo L. Columbo, Massimo Brambilla, Franco Prati, Sandro Dal Cin, Maximilian Beiser, Nikola Opačak, Pawan Ratra, Michael Pushkarsky, David Caffey, Timothy Day, Luigi A. Lugiato, Benedikt Schwarz, and Federico Capasso
Abstract: Despite the ongoing progress in integrated optical frequency comb technology, compact sources of short bright pulses in the mid-infrared wavelength range from 3 μm to 12 μm so far remained beyond reach. The state-of-the-art ultrafast pulse emitters in the mid-infrared are complex, bulky, and inefficient systems based on the downconversion of near-infrared or visible pulsed laser sources. Here we show a purely DC-driven semiconductor laser chip that generates one picosecond solitons at the center wavelength of 8.3 μm at GHz repetition rates. The soliton generation scheme is akin to that of passive nonlinear Kerr resonators. It relies on a fast bistability in active nonlinear laser resonators, unlike traditional passive mode-locking which relies on saturable absorbers or active mode-locking by gain modulation in semiconductor lasers. Monolithic integration of all components - drive laser, active ring resonator, coupler, and pump filter - enables turnkey generation of bright solitons that remain robust for hours of continuous operation without active stabilization. Such devices can be readily produced at industrial laser foundries using standard fabrication protocols. Our work unifies the physics of active and passive microresonator frequency combs, while simultaneously establishing a technology for nonlinear integrated photonics in the mid-infrared.
Physical Review Letters 134, 023802, PDF
Theodore P. Letsou, Dmitry Kazakov, Pawan Ratra, Lorenzo L. Columbo, Massimo Brambilla, Franco Prati, Cristina Rimoldi, Sandro Dal Cin, Nikola Opačak, Henry O. Everitt, Marco Piccardo, Benedikt Schwarz, and Federico Capasso
Abstract: Coupling is an essential mechanism that drives complexity in natural systems, transforming single, noninteracting elements into intricate networks with rich physical properties. Here, we demonstrate a chip-scale coupled laser system that exhibits complex optical states impossible to achieve in an uncoupled system. We show that a pair of coupled semiconductor ring lasers spontaneously forms a frequency comb consisting of the hybridized modes of its coupled cavity, exhibiting a large number of phase-locked tones that anticross with one another. Experimental coherent waveform reconstruction reveals that the hybridized frequency comb manifests itself as pairs of bright and dark picosecond-long solitons circulating simultaneously. The dark and bright solitons exit the coupled cavity at the same time, leading to breathing bright solitons temporally overlapped with their dark soliton counterparts—a state inaccessible for a single, free-running laser. Our results demonstrate that the rules that govern allowable states of light can be broken by simply coupling elements together, paving the way for the design of more complex networks of coupled on-chip lasers.
Nature 625, 685–690 (2024), PDF
Nikola Opačak, Dmitry Kazakov, Lorenzo L. Columbo, Maximilian Beiser, Theodore P. Letsou, Florian Pilat, Massimo Brambilla, Franco Prati, Marco Piccardo, Federico Capasso, and Benedikt Schwarz
Abstract: Optical frequency-comb sources, which emit perfectly periodic and coherent waveforms of light, have recently rapidly progressed towards chip-scale integrated solutions. Among them, two classes are particularly significant—semiconductor Fabry–Perot lasers and passive ring Kerr microresonators. Here we merge the two technologies in a ring semiconductor laser and demonstrate a paradigm for the formation of free-running solitons, called Nozaki–Bekki solitons. These dissipative waveforms emerge in a family of travelling localized dark pulses, known within the complex Ginzburg–Landau equation. We show that Nozaki–Bekki solitons are structurally stable in a ring laser and form spontaneously with tuning of the laser bias, eliminating the need for an external optical pump. By combining conclusive experimental findings and a complementary elaborate theoretical model, we reveal the salient characteristics of these solitons and provide guidelines for their generation. Beyond the fundamental soliton circulating inside the ring laser, we demonstrate multisoliton states as well, verifying their localized nature and offering an insight into formation of soliton crystals. Our results consolidate a monolithic electrically driven platform for direct soliton generation and open the door for a research field at the junction of laser multimode dynamics and Kerr parametric processes.
Nature Communications 15, 607 (2024), PDF
Dmitry Kazakov, Theodore P. Letsou, Maximilian Beiser, Yiyang Zhi, Nikola Opačak, Marco Piccardo, Benedikt Schwarz, and Federico Capasso
Abstract: High-quality optical ring resonators can confine light in a small volume and store it for millions of roundtrips. They have enabled the dramatic size reduction from laboratory scale to chip level of optical filters, modulators, frequency converters, and frequency comb generators in the visible and the near-infrared. The mid-infrared spectral region (3−12 μm), as important as it is for molecular gas sensing and spectroscopy, lags behind in development of integrated photonic components. Here we demonstrate the integration of mid-infrared ring resonators and directional couplers, incorporating a quantum cascade active region in the waveguide core. It enables electrical control of the resonant frequency, its quality factor, the coupling regime and the coupling coefficient. We show that one device, depending on its operating point, can act as a tunable filter, a nonlinear frequency converter, or a frequency comb generator. These concepts extend to the integration of multiple active resonators and waveguides in arbitrary configurations, thus allowing the implementation of purpose-specific mid-infrared active photonic integrated circuits for spectroscopy, communication, and microwave generation.
Nature Communications 14, 3960 (2023), PDF
Mohamed ElKabbash*, Nathaniel Hoffman*, Andrew R. Lininger*, Sohail A. Jalil, Theodore P. Letsou, Michael Hinczewski, Giuseppe Strangi, and Chunlei Guo
Abstract: Structural coloring is a photostable and environmentally friendly coloring approach that harnesses optical interference and nanophotonic resonances to obtain colors with a range of applications including display technologies, colorful solar panels, steganography, décor, data storage, and anticounterfeiting measures. We show that optical coatings exhibiting the photonic Fano Resonance present an ideal platform for structural coloring; they provide full color access, high color purity, high brightness, controlled iridescence, and scalable manufacturing. We show that an additional oxide film deposited on Fano resonant optical coatings (FROCs) increases the color purity (up to 99%) and color gamut coverage range of FROCs to 61% of the CIE color space. For wide-area structural coloring applications, FROCs have a significant advantage over existing structural coloring schemes.
Communications Physics 6, 69 (2023), PDF
Yasaman Kiasat*, Maria Grazia Donato*, Michael Hinczewski*, Mohamed ElKabbash, Theodore Letsou, Rosalba Saija, Onofrio Maria Maragò, Giuseppe Strangi, and Nader Engheta
Abstract: Optomechanics deals with the control and applications of mechanical effects of light that stems from the redistribution of photon momenta in light scattering. As an example, light-induced levitation of an infinitesimally small dipolar particle is expected in front of epsilon-near-zero (ENZ) metamaterials. However, a theoretical understanding of these effects on single-material and multi-material larger particles is still lacking. Here, we investigate, analytically and numerically, optical forces on polarizable particles with size ranging from 20 nm to a 1 μm in proximity of ENZ metamaterials. We look at the general features of the repulsive-attractive optomechanics from the nano to the microscale exploiting different theoretical methods (dipole approximation, finite elements calculations, transition (T-)matrix). We discuss the role of realistic layered materials, as our ENZ substrate, on optical forces and analyze the influence of composition and shape by studying a range of complex particles (dielectric, core-shell, plasmonic ellipsoids). Physical insights into the results are discussed and future research directions are forecasted. Our results provide possibilities in exploiting engineered materials and surfaces for the manipulation and tailoring of light-induced forces in optomechanics.
Nature Nanotechnology 16, 440–446 (2021), PDF
Mohamed ElKabbash, Theodore P. Letsou, Sohail A. Jalil, Nathaniel Hoffman, Jihua Zhang, James Rutledge, Andrew R. Lininger, Chun-Hao Fann, Michael Hinczewski, Giuseppe Strangi, and Chunlei Guo
Abstract: Optical coatings are integral components of virtually every optical instrument. However, despite being a century-old technology, there are only a handful of optical coating types. Here, we introduce a type of optical coatings that exhibit photonic Fano resonance, or a Fano-resonant optical coating (FROC). We expand the coupled mechanical oscillator description of Fano resonance to thin-film nanocavities. Using FROCs with thicknesses in the order of 300 nm, we experimentally obtained narrowband reflection akin to low-index-contrast dielectric Bragg mirrors and achieved control over the reflection iridescence. We observed that semi-transparent FROCs can transmit and reflect the same colour as a beam splitter filter, a property that cannot be realized through conventional optical coatings. Finally, FROCs can spectrally and spatially separate the thermal and photovoltaic bands of the solar spectrum, presenting a possible solution to the dispatchability problem in photovoltaics, that is, the inability to dispatch solar energy on demand. Our solar thermal device exhibited power generation of up to 50% and low photovoltaic cell temperatures (~30 °C), which could lead to a six-fold increase in the photovoltaic cell lifetime.
Abstract: Quantum cascade lasers (QCLs) have been the dominant source of high-power infrared radiation ever since their invention in 1994. The ability to engineer their emission wavelengths from 3 𝜇m to 300 𝜇m has allowed scientists to use QCLs in a plethora of applications, ranging from spectroscopy to tomography. In addition, QCLs are highly non-linear devices, and possess the ability to emit many frequencies of light simultaneously. This has made them excellent candidates for frequency combs, which are broadband light sources that emit equally-spaced frequencies with a well-defined phase relation. By manipulating the optical non-linearities through dispersion engineering, QCLs can be made to enter frequency combs states on-demand. By mixing two different frequency combs, absorption features at optically frequencies can be encoded into the radio-frequency domain, eliminating the need for expensive, high-frequency detectors. This "dual-comb spectrometer" offers a chip-scale alternative to bulky spectrometers, making it one of the most attractive applications of QCLs. This thesis outlines the development, characterization and theory of QCL frequency combs operating in the atmospheric transmission window (8 𝜇m – 12 𝜇m)—a spectral region where many chemical species have their fundamental vibrational and absorption bands. By borrowing techniques commonly used in ultra-fast optics, the dispersion of QCLs—which is the primary catalyst for comb formation—can be tuned without the use of mechanically-moving parts. In addition, this thesis utilizes optical coherence techniques to reconstruct the electric field profile of QCL combs, which provides valuable insight on the physics of their formation.
Meas. Sci. Technol. 31 115201 (2020), PDF
Mohamed ElKabbash, Kandammathe Valiyaveedu Sreekanth, Arwa Fraiwan, Jonathan Cole, Yunus Alapan, Theodore P. Letsou, Nathaniel Hoffman, Chunlei Guo, R Mohan Sankaran, and Umut A Gurkan
Abstract: We demonstrated an optically-active antireflection, light absorbing, optical coating as a hydrogen gas sensor. The optical coating consists of an ultrathin 20 nm thick palladium film on a 60 nm thick germanium layer. The ultrathin thickness of the Pd film (20 nm) mitigates mechanical deformation and leads to robust operation. The measurable quantities of the sensors are the shift in the reflection minimum and the change in the full width at half maximum of the reflection spectrum as a function of hydrogen gas concentration. At a hydrogen gas concentration of 4%, the reflection minimum shifted by ∼46 nm and the FWHM increased by ∼228 nm. The sensor showed excellent sensitivity, demonstrating a 6.5 nm wavelength shift for 0.7% hydrogen concentration, which is a significant improvement over other nanophotonic hydrogen sensing methods. Although the sensor's response showed hysteresis after cycling hydrogen exposure, the sensor is robust and showed no deterioration in its optical response after hydrogen deintercalation.
ACS Photonics 2019, 6, 8, 1889–1894, PDF
Mohamed ElKabbash, Kandammathe V Sreekanth, Yunus Alapan, Myeongseop Kim, Jonathan Cole, Arwa Fraiwan, Theodore Letsou, Yandong Li, Chunlei Guo, R Mohan Sankaran, Umut A Gurkan, Michael Hinczewski, Giuseppe Strangi
Abstract: Hydrogen sensing is important in many industrial, biomedical, environmental, and energy applications. Realizing a practical, reliable, and inexpensive hydrogen sensor, however, is an ongoing challenge. Here, we demonstrate hydrogen sensing based on an optically active metal–dielectric–metal (MDM) perfect light absorber. The cavity enables perfect broadband light absorption (>99.999%) with optical losses localized in an ultrathin palladium (Pd) layer. Upon exposure to hydrogen, the Pd layer forms a hydride which actively shifts the cavity minimum reflectance wavelength by ∼60 nm for a hydrogen concentration of 4%. The sensor enjoys extremely high figure of merit. The ease of fabrication, large area, and high sensitivity of our sensor make it an attractive and practical option, especially for miniaturized hydrogen sensors vital for medical and food safety applications.
ACS Photonics 2019, 6, 7, 1610–1617, PDF
Kandammathe Valiyaveedu Sreekanth, Mohamed ElKabbash, Rohit Medwal, Jihua Zhang, Theodore P. Letsou, Giuseppe Strangi, Michael Hinczewski, Rajdeep S Rawat, Chunlei Guo, and Ranjan Singh
Abstract: The generalized Brewster angle (GBA) is the incidence angle at polarization by reflection for p- or s-polarized light. Realizing an s-polarization Brewster effect requires a material with magnetic response, which is challenging at optical frequencies since the magnetic response of materials at these frequencies is extremely weak. Here, we experimentally realize the GBA effect in the visible using a thin-film absorber system consisting of a dielectric film on an absorbing substrate. Polarization by reflection is realized for both p- and s-polarized light at different angles of incidence and multiple wavelengths. We provide a theoretical framework for the generalized Brewster effect in thin-film light absorbers. We demonstrate hydrogen gas sensing using a single-layer graphene film transferred on a thin-film absorber at the GBA with ∼1 fg/mm2 aerial mass sensitivity. The ultrahigh sensitivity stems from the strong phase sensitivity near the point of darkness, particularly at the GBA, and the strong light–matter interaction in planar nanocavities. These findings depart from the traditional domain of thin films as mere interference optical coatings and highlight its many potential applications including gas sensing and biosensing.
Optical Materials Express 9, 3, 1386-1393 (2019), PDF
Theodore P. Letsou, Mohamed ElKabbash, Shamreen Iram, Michael Hinczewski, and Giuseppe Strangi
Abstract: Heating reflective metals is known to produce a wide range of colors due to oxidation of the metal surface. In fact, the most vibrant colors used in the pre-industrial era came from oxides, acetates and carbonates of metal ores and minerals. In this work, we show that heating low reflectivity metals, e.g., Ni and Ti, creates structural colors through perfect light absorption. We tune the absorption across the visible and NIR spectrum by changing the heating duration and, consequently, the oxide thickness. We demonstrate experimentally angle-insensitive perfect and near-perfect absorption in the visible and NIR regimes up to ±60∘. The absorption is insensitive to the incidence angle due to the relatively high refractive index of the formed oxides, which create iridescent free coloration. We demonstrate that the oxide layer thickness, with refractive index n, is <𝜆/4𝑛 due to non-trivial phase change at the oxide/metal interfaces, which makes these systems the simplest example of meta-surfaces based on thin films. The results show that oxidized metals can have applications beyond producing vibrant colors.
Advanced Optical Material 6, 22 (2018) 1800672, PDF
Mohamed ElKabbash, Shamreen Iram, Theodore P. Letsou, Michael Hinczewski, and Giuseppe Strangi
Abstract: Optical absorbers comprised of an ultrathin lossy dielectric film on an opaque metallic substrate are an attractive alternative to lithographically intense metamaterial and nanoplasmonic optical absorbers as they allow for large-scale, cost-effective fabrication. However, requiring that the dielectric is lossy and the metallic substrate is highly reflective but not a perfect electric conductor (PEC) limits the wavelength range and materials that can be used to realize strong to perfect light absorption. In this work, we theoretically and experimentally investigate light absorption using ultrathin lossless dielectric films. By choosing proper lossless ultrathin dielectrics and substrates, iridescence free, perfect light absorption is possible over the visible, near infrared (NIR), and short-wave infrared (SWIR) wavelength ranges with designer absorption properties. The proposed class of ultrathin film absorbers relaxes many constraints on the type of materials used to realize perfect light absorption. The flexibility of our design makes it relevant for many applications specifically in structural coloring, selective thermal emission, thermo-photovoltaics, photo-thermoelectric generation, and gas sensing.
Optics Letters 42, 18 (2017) 3598-3601, PDF
Mohamed ElKabbash, Efe Ilker, Theodore P. Letsou, Nathaniel D. Hoffman, Alexander Yaney, Michael Hinczewski, and Giuseppe Strangi
Abstract: Perfect light absorption in the visible and near-infrared (NIR) was demonstrated using metamaterials, plasmonic nanostructures, and thin films. Thin film absorbers offer a simple and low-cost design as they can be produced on large areas and without lithography. Light is strongly absorbed in thin film metal-dielectric-metal (MDM) cavities at their resonance frequencies. However, a major drawback of MDM absorbers is their strong resonance iridescence, i.e., angle dependence. Here, we solve the iridescence problem by achieving angle-insensitive narrowband perfect and near-perfect light absorption. In particular, we show analytically that using a high-index dielectric in MDM cavities is sufficient to achieve angle-insensitive cavity resonance. We demonstrate experimentally angle-insensitive perfect and near-perfect absorbers in the NIR and visible regimes up to ±60°. By overcoming the iridescence problem, we open the door for practical applications of MDM absorbers at optical frequencies.
